Vertical-current MOSFET devices are used in various applications, such as DC/DC converters, devices for control and protection of batteries and lamps, and products for the automotive sector.
In particular, in the latter application, devices are typically required that are able to dissipate low amounts of heat even when they operate in high-current conditions. In practice, the device typically must present low source-drain on-resistance (Rdson), as well as the ability of withstanding a high reverse-biasing voltage (high BVdss).
In vertical-current devices of a planar type, the requisites corresponding to the two parameters referred to above (Rdson and BVdss) are in conflict in so far as to obtain a high reverse voltage it is necessary to increase the epitaxial thickness and/or to increase the resistivity of the epitaxial layer. In both cases, there is an increase in the Rdson since an increase in thickness determines a longer current path in the on state, and a greater resistivity of the epitaxial layer involves a higher resistance to the flow of current.
To reduce the source-drain on resistance it is possible to use a column structure that enables an increase in the body-drain perimeter so as to exploit the entire volume of the epitaxial layer. This technique enables the use of a more heavily doped epitaxial layer, thus one having lower resistivity, for a same reverse voltage, reducing the component of the Rdson due to the epitaxial layer (defined hereinafter as “epitaxial on resistance Repi”).
An embodiment of a device having a column structure is illustrated in FIG. 1. In particular, FIG. 1 regards a device 1 of N-channel type having an epitaxial layer 3 of N type housing columns 2 of P type underneath body regions 4. Source regions 5 are formed within the body regions 4, and gate regions 6, of polysilicon, extend on top of the epitaxial layer 3, separated therefrom by respective gate-oxide layers 7. A metal region 8 electrically couples the source regions 5 and the body regions 4, and is electrically insulated from the gate regions 6 by insulating regions 9.
The columns 2 extend in a continuous way in the direction perpendicular to the plane of the drawing, for the entire length of the device, parallel to the body regions 4, to form strips or walls as illustrated in FIG. 2.
Embodiments of MOSFET devices with columnar structures are described in U.S. Pat. No. 6,630,698, US 2002/14671, and U.S. Pat. No. 6,586,798, which are incorporated by reference.
In devices with a columnar structure, it may be possible to obtain balance or compensation of charge between the dopant of the columns 2, of P type, and the charge of the epitaxial layer 3, of N type, so that the total charge of the columns 2 is equal and of opposite sign with respect to the total charge of the epitaxial layer 3. This condition involves complete depletion of the free carriers both in the epitaxial layer 3 and in the columns 2 so as to form a carrier-free area, which, behaving like an insulating layer, enables high values of reverse (breakdown) voltage, with an electrical field of almost uniform extension both in modulus and in direction through the entire region comprising the epitaxial layer 3 and the columns 2. In particular, it may be possible to bias the device so that the electrical field is close to the critical electrical field, which is the maximum electrical field that a PN junction can withstand at the interface, beyond which the process of avalanche conduction (breakdown) is triggered.
Using the principle of charge balance, it may thus be possible to choose a high dopant concentration in the epitaxial layer 3, appropriately balanced by the dopant in the columns 2. This choice may have, however, limits due to the need of calibrating the intercolumnar distance for ensuring complete depletion of the entire epitaxial region, including the columns 2. This distance depends upon the lithographic resolution obtainable with the specific used technology.
Thanks to the configuration of the strips that form the columns 2 (visible in FIG. 2), the current flow, indicated by the arrows 10 in the on state of the device 1, is thus confined between two contiguous columns 2 in conditions of partial depletion, as occurs in conduction.
The value of the epitaxial on-resistance Repi is thus determined by the columnar geometry, and hence by the volume of the epitaxial layer 3 traversed by the current flow comprised between two contiguous columns 2.